UPDATE 10.10AM: The B-double involved in a substance leak on the Bruce Highway will undergo maintenance as part of an investigation.

Emergency crews were called to the leak, believed to be potentially harmful ammonia, on Monday night.

The company of the vehicle was able to depressurise gas from the tanker after the leak.

Maryborough Fire Station Officer Adrian Massingham said an investigation of the vehicle would be undertaken.

He praised the driver of the truck for leaving the highway to minimise any traffic disruptions.

"If it was left on the highway there would have been road closures," Officer Massingham said.

UPDATE: Two fire-fighters are approaching a B-double leaking potentially hazardous material to conduct atmospheric testing that will help emergency crews determine the safest way to manage the incident.

Police and ambulance crews are still at the scene after the truck was parked in a field on the Bruce Highway on the Glenorchy Straight after it was found to be leaking a potentially dangerous substance, believed to be ammonia.

A spokeswoman from the Queensland Fire and Emergency Service said the two fire-fighters were dressed in protective equipment.

She said the testing would help crews find out the nature of the substance, whether it was a safe levels and what was going on with it.

EARLIER: Police are maintaining an 800m exclusion zone around a truck leaking a potentially dangerous substance on the Bruce Highway south of Maryborough.

The B-double is in a field near Old Gympie Rd on the Glenorchy Straight.

The highway may close as crews work to control the incident.

Police, fire and ambulance crews are at the scene.

The substance is believed to be ammonia.

A spokeswoman from the Queensland Fire and Emergency Service said three fire crews were currently at the scene.

She said the incident stated about 2.45pm on Monday.

The spokeswoman said police would be following procedural guidelines when it came to setting the exclusion zone around the vehicle.